Photomasks, or reticles, are commonly used for photolithography in semiconductor manufacturing. Photomasks are typically made from very flat pieces of quartz or glass with a layer of chromium deposited on one side. The pattern being used to transfer an image to a wafer on BIM (Binary Mask) or PSM (Phase Shift Mask) masks during a photolithography processing. While contamination of photomasks has always been a problem, high precision masks, such as are used in photolithography having wavelengths equal to or less than 248 nm, are particularly susceptible to defects.
One type of photomask contamination is referred to as haze contamination. Haze contamination is a precipitant formed from mask cleaning chemical residual or impurity of fab or tool environment cross exposure. For example, when a solution including ammonium (NH4) and sulfate (SO4) is used to clean a photomask, contamination becomes apparent when the photomask is exposed to a short wavelength UV light, such as 248 or 193 nm.
One interesting aspect of the frame that has evolved over the years is the addition of a vent hole (also called a pressure relief valve, or PRV). Before the vent hole was added, photomasks shipped via air would experience pellicle inflation during high altitude (low pressure). The membrane would “puff up,” sometimes to the point of making contact with the box lid in which it was stored. This contact would produce a scuff that rendered the mask useless before it ever made it into the wafer fabrication cleanroom. To alleviate this problem, a small hole was drilled into the frame allowing the pressure to equalize between the encapsulated air under the pellicle membrane and the ambient air. Since the statistical change that a small particle could in fact be sucked through the hole during pressure equalization is nonzero, holes can either be covered with a filter material or be cut as a tortuous path through the frame, or both. (this may be for FIG. 2)
UV laser applied on the mask with pellicle creates high energy environment in the space trapped between the pellicle and the mask surface. This environment is filled with outgassed chemicals from pellicle materials and highly reactive oxygen radicals and ozone produced during the laser exposure, creating an ideal nest for photochemical reactions that could possibly induce the haze defect formation on the mask surface.